This invention relates to a film-type lithium secondary battery and its manufacturing method, and in details to an improvement in an electrode of the film-type lithium secondary battery and an improving method for the electrode.
In recent years, portable devices such as a portable telephone, a PHS and a small personal computer etc. are undergoing remarkable development in fabrication into small-size and light-weight with a progress of electronics technology. Further, batteries serving as power supplies for use in these portable devices are also required to be built into small-size and light-weight form.
A lithium battery can be mentioned here as an example of a battery to be expected for use in such purpose. In addition to a lithium primary battery already put in practical use, studies have been made on the lithium secondary battery to be put it in practical use, and to achieve its high capacity and long service life.
Almost all of the foregoing various lithium batteries are of cylindrical type. While, in the lithium primary battery, a film-type form is also put in practical use by a manufacturing method for which a solid electrolyte is used and a printing technology is applied. Utilizing this method, many studies have been made to put the film-type form into practical use in fields of the lithium secondary battery and a lithium ion secondary battery, too.
The cylindrical lithium secondary battery is made up through a process in which an electrode group composed of a positive electrode, a negative electrode and a separator is inserted in a cylindrical container and then a liquid electrolyte is filled in it. On the other hand, the film-type lithium secondary battery is made up through a process in which the positive electrode, the negative electrode and the separator composed of a solid-state or a gel-state electrolyte are made up respectively and then laminated one on the other. However, the film-type lithium secondary battery of this type has included such defects as a poor high-rate charge/discharge property and a short cycle life.
The following two points may be considered as reasons for the above defects.
{circle around (1)} In case of the cylindrical battery, the electrode group composed of the positive electrode, the negative electrode and the separator is inserted in the cylindrical container and the liquid electrolyte is filled in the container, so that it is easy to control electronic isolation of an electrode active material due to swelling of the liquid electrolyte by applying a potential to the electrode group. In case of the film-type battery, however, the positive electrode and the negative electrode are opposed each other through the electrolyte, so that it is difficult to apply a potential to the electrode.
{circle around (2)} Since distribution of the electrolyte in the electrode become not uniform, a degree of transfer of lithium ion in the electrode is small. In addition, since fine corrugations remain on electrode surfaces, a surface resistance between the electrode and the separator is large.
This invention is made in consideration of the above-mentioned problems, and an object of this invention is to provide a film-type lithium secondary battery which includes a small surface resistance of the battery inside and therefore is able to offer a high and stable battery performance, and to provide a manufacturing method by which such a film-type lithium secondary battery can be obtained easily.
A film-type lithium secondary battery of this invention provides a film-type lithium secondary battery in which at least a positive electrode and a negative electrode are installed, the positive electrode and the negative electrode are composed by coating an electrode composite onto a current collector respectively, and the electrode composite contains at least an electrode-active material and a solid-state or a gel-state electrolyte; characterized by that, in at least the positive electrode among the positive and negative electrodes, an electrolyte layer composed only of an electrolyte integrated with the electrolyte in the electrode composite is formed on a surface of the electrode composite, and the positive electrode is opposed against the negative electrode through the electrolyte layer.
In the film-type lithium secondary battery of this invention, since the corrugated surface of the electrode composite is covered with the electrolyte layer, the surface resistance between the electrode and the electrolyte layer is reduced by a large margin. Consequently, the film-type lithium secondary battery of this invention becomes superior in an initial capacity, a high-rate charge/discharge property and a cycle life characteristic etc.
In the film-type lithium secondary battery of this invention, following structures (1) to (5) may be used.
(1) The positive electrode is opposed against the negative electrode only through the electrolyte layer.
In this structure, since the electrolyte of the electrolyte layer is integrated with the electrolyte in the electrode composite, the electrolyte layer has a sufficient mechanical strength. Therefore, the electrolyte layer can serve as a function of the separator so that another separator becomes unnecessary. In addition, the positive electrode becomes in contact with the negative electrode only through their electrolyte layers. As the result, an inside resistance of battery is reduced further. Consequently, the film-type lithium secondary battery of this invention becomes superior in the initial capacity, the high-rate charge/discharge property and the cycle life characteristic etc.
(2) The positive electrode is opposed against the negative electrode through the electrolyte layer and the separator.
In this structure, a contact between the electrode and the separator means a contact between the electrolytes because it means a contact between the electrolyte layer and the separator. Consequently, the film-type lithium secondary battery of this invention becomes superior in the initial capacity, the high-rate charge/discharge property and the cycle life characteristic etc.
(3) A total thickness of the electrolyte layer ranges from 2 xcexcm to 100 xcexcm.
According to this structure, an effect gained by forming the electrolyte layer on the surface of electrode composite can be obtained effectively. In other words, when the total thickness of the electrolyte layer is smaller than 2 xcexcm, the reduction in the surface resistance between the electrode and the electrolyte layer becomes insufficient because there is a possibility that the corrugation of the surface of electrode composite would not be covered completely. When the total thickness of the electrolyte layer is larger than 100 xcexcm, a bulk resistance of the electrolyte layer becomes large. Therefore, in any case there is a possibility that the high-rate charge/discharge property and the cycle life characteristic would not be improved. Especially, when the positive electrode is opposed against the negative electrode only through the electrolyte layer and the total thickness of the electrolyte layer is smaller than 2 xcexcm, an inside short-circuiting is apt to occur because there is a possibility that the corrugation of surface of the electrode composite would not be covered completely. Therefore, this structure is not preferable.
(4) In at least the positive electrode among the positive and negative electrodes, the electrode composite contains the binder, and the electrolyte is distributed uniformly in the electrode composite while maintaining a binding ability provided by the binder.
In this structure, a bulk density of the electrode active material in the electrode composite can be improved, because the binder is contained in the electrode composite so as to maintain the binding ability provided by the binder. In addition, the degree of transfer of lithium ion in the positive electrode can be improved because the electrolyte is distributed uniformly in the electrode composite. Consequently, the film-type lithium secondary battery of this invention becomes superior in the initial capacity, the high-rate charge/discharge property and the cycle life characteristic etc.
In this case, it is preferable to use polyvinylidene fluoride, propylene hexafluoride, or a copolymer of polyvinylidene fluoride and propylene hexafluoride, as the binder.
When the above-mentioned binder is used, a binding ability between electrode active material particles and a binding ability between the electrode composite and the current collector can be obtained sufficiently in order to maintain the electrode property. In addition, a harmful influence of the binder on the electrode reaction can be prevented.
(5)
In at least the positive electrode among the positive and negative electrodes, an organic polymer composing the electrolyte of the electrodeicomposite has both structures providing a high affinity and a low affinity for a liquid electrolyte formed by dissolving an electrolyte salt composing the electrolyte into a plasticizer.
In this structure, since the structures including the high affinity and low affinity for the liquid electrolyte coexist at least in the organic polymer of the electrode composite of the positive electrode, the structures providing the high affinity and low affinity for the liquid electrolyte are phase isolated into micron unit in the organic polymer. For this reason, at least a liquid holding ability of the positive electrode is maintained and a state where the transfer of lithium ion is not prohibited can be realized. On the other hand, since the organic polymer of the separator has a major structure including a high affinity for the liquid electrolyte, so that it has a property to easily restrict the liquid electrolyte. Therefore, when a transfer of the liquid electrolyte occurs due to a transfer of lithium ion at time of charging and discharging, the liquid electrolyte is easily restricted in the separator. However, the lithium ion transfers more easily in the organic polymer of the positive electrode than in the organic polymer of the separator. Therefore, even if the transfer of electrolyte occurs due to the transfer of lithium ion at time of charge/discharge, the restriction of liquid electrolyte in the separator can be controlled, a sufficient amount of the liquid electrolyte can be held in both the positive and negative electrode composites even after progress of charge/discharge cycle, and thus a reduction in the capacity due to the progress of charge/discharge cycle can be controlled.
A manufacturing method of a film-type lithium secondary battery of a first invention of this application is characterized in that at least a positive electrode and a negative electrode are installed, the positive electrode and the negative electrode are composed by coating an electrode composite onto a current collector respectively, and the electrode composite contains at least an electrode active material and a solid-state or a gel-state electrolyte; characterized by that at least the positive electrode among the positive and negative electrodes is made up through following processes (a) to (c), and the positive electrode is opposed against the negative electrode through an electrolyte layer obtained by the following process (c).
(a) a sheet forming process in which at least the electrode active material is mixed in an organic solvent, the mixed solution is coated on the current collector, dried and pressed, so as to form an electrode active material sheet;
(b) an impregnation process in which the electrode active material sheet is dipped in an electrolytic solution prepared by mixing at least an electrolyte salt with an organic monomer having two or more polymeric functional groups at its chain ends, so that the electrolytic solution is impregnated to the electrode active material sheet, and the electrolytic solution is made exist on a surface of the electrode active material sheet in a form of a liquid film;
(c) a polymerizing process in which the organic monomer in the electrolyte is polymerized to form an organic polymer so that the electrolyte in the electrode active material sheet is brought into a solid-state or a gel-state, and an electrolyte layer composed only of the solid-state or the gel-state electrolyte is formed on the surface of the electrode active material sheet.
In the manufacturing method of the first invention, the electrolyte can be distributed uniformly in the electrode active material sheet because the electrolytic solution is impregnated to the electrode active material sheet. In addition, the solid-state or the gel-state electrolyte layer can be formed on the surface of electrode composite because the organic polymer is formed under the state where the electrolytic solution is made exist on the surface of the electrode active material sheet in the form of the liquid film. Therefore, the film-type lithium secondary battery of this invention can be obtained surely.
In the manufacturing method of the first invention, following processes (1) to (3) may be used further.
(1) In the impregnation process, the dipping of the electrode active material sheet into the electrolytic solution is carried out under an ambient pressure reduced from the atmospheric pressure.
According to this process, the electrolyte can be impregnated sufficiently even when a time of impregnation process is short. Therefore, a battery manufacturing process time can be shortened and a production cost can be minimized.
A value of reduction pressure preferably ranges from 0.03 kPa to 15 kPa. Thereby, the electrolyte can be impregnated surely and sufficiently even when the time of impregnation process is short, so that a sufficient initial capacity can be obtained.
(2) In the impregnation process, the dipping of the electrode active material sheet into the electrolytic solution is carried out under an ambient pressure reduced from the atmospheric pressure and then under an ambient pressure increased from the atmospheric pressure.
According to this process, the electrolyte can be impregnated sufficiently even when the time of impregnation process is further shorter than that of the process (1). Therefore, the battery manufacturing process time can be shortened and the production cost can be minimized.
A value of reduction pressure preferably ranges from 0.1 kPa to 15 kPa, and a value of increased pressure is preferably smaller than or equal to 400 kPa. Thereby, the electrolyte can be impregnated surely and sufficiently even when the time of impregnation process is further shorter than that of the process (1), so that a sufficient initial capacity can be obtained.
(3) In the above processes (1) and (2), a process may be used, in which the electrode active material sheet is put in a closed pressure vessel, a pressure in this vessel is reduced from the atmospheric pressure, and then the electrolytic solution is thrown in the closed pressure vessel.
According to this process, the impregnation process can be carried out with a good workability.
A manufacturing method of a film-type lithium secondary battery of a second invention of this application is characterized in that at least a positive electrode and a negative electrode are installed, the positive electrode and the negative electrode are composed by coating an electrode composite onto a current collector respectively, and the electrode composite contains at least an electrode active material and a solid-state or a gel-state electrolyte; characterized by that at least the positive electrode among the positive and negative electrodes is made up through following processes (a) to (c), and the positive electrode is opposed against the negative electrode through an electrolyte layer obtained by the following process (c).
(a) a sheet forming process in which at least the electrode active material is mixed in an organic solvent, the mixed solution is coated on the current collector, dried and pressed, so as to form an electrode active material sheet;
(b) a coating process in which an electrolytic solution prepared by mixing at least an electrolyte salt with an organic monomer having two or more polymeric functional groups at its chain ends, is coated on a surface of the electrode active material sheet, so that the electrolytic solution is permeated into the electrode active material sheet and the electrolytic solution is made exist on a surface of the electrode active material sheet in a form of a liquid film;
(c) a polymerizing process in which the organic monomer in the electrolyte is polymerized to form an organic polymer so that the electrolyte in the electrode active material sheet is brought into a solid-state or a gel-state, and an electrolyte layer composed only of the solid-state or the gel-state electrolyte is formed on the surface of the electrode active material sheet.
In the manufacturing method of the second invention, the solid-state or the gel-state electrolyte layer can be formed on the surface of the electrode composite because the organic polymer is under the state where the electrolytic solution is made exist on the surface of the electrode active material sheet in the form of the liquid film. Therefore, the film-type lithium secondary battery of this invention can be obtained surely. Further, its work can be done easily because the electrolytic solution is coated on the surface of the electrode active material sheet.
A manufacturing method of a film-type lithium secondary battery of a third invention of this application is characterized by that at least a positive electrode and a negative electrode are installed, the positive electrode and the negative electrode are composed by coating an electrode composite onto a current collector respectively, and the electrode composite contains at least an electrode active material and a solid-state or a gel-state electrolyte; characterized in that at least the positive electrode among the positive and negative electrodes is made up through following processes (a) to (d), and the positive electrode is opposed against the negative electrode through an electrolyte layer obtained by the following process (d).
(a) a mixing process to obtain a mixture prepared by mixing at least an electrode active material and an electrolyte salt and an organic monomer having two or more polymeric functional groups at its chain ends;
(b) an sheet forming process in which the mixture is coated on the current collector to form a mixture sheet;
(c) a shelf-leaving process in which the mixture sheet is left on a shelf and the electrode active material in the mixture sheet is settled so as to make the electrolytic solution exist on a surface of the mixture sheet in a form of a liquid film;
(d) a polymerizing process in which the organic monomer in the electrolyte is polymerized to form an organic polymer so that the electrolyte in the mixture sheet is brought into a solid-state or a gel-state, and an electrolyte layer composed only of the solid-state or the gel-state electrolyte is formed on a surface of the mixture sheet.
In the manufacturing method of the third invention, the electrolyte can be distributed uniformly in the mixture sheet because the electrolyte is mixed with the electrode active material. In addition, the solid-state or the gel-state electrolyte layer can be formed on the surface of the electrode composite because the organic polymer is formed under the state where the electrolytic solution is made exist on the surface of the mixture sheet in the form of the liquid film. Therefore, the film-type lithium secondary battery of this invention can be obtained surely.
In the manufacturing methods of the first through third inventions, following processes (1) to (4) may be used further.
(1) A mold releasing film is covered on the surface of the electrode active material sheet or the surface of the mixture sheet with a clearance of desired thickness left between them, so as to make the electrolytic solution exist in the clearance in a form of a liquid film.
According to this process, a thickness of the electrolyte layer can be set to a desired value because the thickness of the electrolyte layer can be controlled by the thickness of the clearance.
(2) A binder is mixed in the process (a).
According to this process, since the electrode active material etc. are pressed with the binder contained in it, a bulk density of the electrode active material can be improved in the electrode active material sheet or the mixture sheet, and in addition the binding ability between the active material particles and that between the electrode composite and the current collector can be maintained by the binder. Therefore, the film-type lithium secondary battery of this invention can be obtained surely.
In this case, it is preferable to use polyvinylidene fluoride, propylene hexafluoride, or a copolymer of polyvinylidene fluoride and propylene hexafluoride, as the binder.
(3) In at least the positive electrode among the positive and negative electrodes, materials having structures providing both high affinity and low affinity for a liquid electrolyte formed by dissolving an electrolyte salt composing the electrolyte in a plasticizer, are used for the organic monomer forming a raw material of the organic polymer composing the electrolyte of the electrode composite.
According to this process, the film-type lithium secondary battery of this invention in which a decrease in capacity with a progress of charge/discharge cycle can be controlled, can be obtained surely.
(4) The positive electrode is opposed against the negative electrode through the separator composed of the solid-state or the gel-state electrolyte and the electrolyte layer.
According to this structure, the film-type lithium secondary battery of this invention can be obtained too.